Manufacturers of integrated circuit (IC) devices use test systems or testers to weed out defective devices at various points in a manufacturing process. Broadly, there are two types of digital testers, those suitable for testing memory arrays or circuits, such as flash memory or Random Access Memories (RAM), and those suitable for testing logic circuits, such as Micro Controllers, Application Specific ICs (ASICs), and Programmable Logic Devices (PLDs). Common types of flash memory devices include NOR Flash, NAND Flash, and Serial Data Flash memories.
Flash memory devices are often produced with various data organizations. As part of a typical manufacturing test process, both algorithmic and non-algorithmic data patterns may be programmed into the device. Furthermore, customers often want specific codes, such as a cell phone operating system, programmed into the devices by the manufacturer before receiving the devices. Data patterns or customer specific codes are programmed at specific locations in the Flash memory device with each data word identified by a row address and a column address.
A NAND Flash memory device has storage cells internally organized as rows of data. Each row typically has 512 user programmable cells, or data bits, and ten to twenty additional bits for use by the device manufacturer to store device characteristics and defect information. Usually, rows are grouped together into “blocks,” with each block typically having eight to sixty-four rows. User data is programmed into the NAND Flash rows in a serial fashion by loading a high-speed shift register that mirrors a row, and then simultaneously transferring the data from the shift register into all data bits in the row.
Monolithic NAND Flash devices often contain as many as two billion individual storage locations or cells. With this big a number of storage locations on one integrated circuit, it is often difficult to manufacture perfect devices with 100% functional storage locations. For this reason, extra rows of storage cells are designed into Flash memories in such a way that they can be used to replace defective or otherwise inoperable storage cells. Replacement of defective cells is usually done at the block level with a good replacement block being substituted for a block with defective cells. In the IC industry, the extra cells are called “redundant elements” or “redundant cells” and the repair of a defective device is called “redundancy repair”.
When a defective block is detected in a testing process, the address of the defective block is entered in a “bad block table,” which is stored in designated locations of the Flash memory device. The bad block table can later be used to locate defective storage elements. From this point in the device manufacturing flow, the defective block is no longer programmed with test data patterns or customer codes. If there are more defective blocks than there are redundant blocks, the device is considered “unrepairable” and is discarded.